Cardiac glycosides increase the force of myocardial contraction and inhibit sarcolemmal Na,K-ATPase. However, the relationship between these two events is not well understood. Our previous studies suggest that a moderate enzyme inhibition by the glycosides enhances the intracellular sodium transient associated with membrane depolarization, and the calcium transient by stimulating the sodium/calcium exchange reaction, resulting in an enhanced force of contraction. A greater enzyme inhibition seems to cause cellular sodium accumulation, oscillatory after-potentials, and hence arrhythmias. The objective of this project is to determine whether there is a causal relationship between Na,K-ATPase inhibition by cardiac glycosides and the inotropic (therapeutic) and/or arrhythmogenic (toxic) actions of these agents. This will be achieved by studying the quantitative relationships among enzyme inhibition, sodium pump inhibition (as monitored by ouabain-sensitive rubidium uptake), transient and/or cumulative changes in intracellular sodium ion concentration, and inotropic or toxic effects in isolated heart preparations produced by various Na,K-ATPase inhibitors including cardiac glycosides, or by increased sodium influx rates in several species of animals with different digitalis sensitivities. Intracellular sodium transients, and their enhancement by the above inotropic interventions, will be experimentally demonstrated. The differences in the sequence of biochemical events which link Na,K-ATPase inhibition to inotropic and toxic effects, respectively, will be explored. These studies should provide us with a rational basis for treating digitalis toxicity and designing and testing safer cardiotonic drugs, as well as promote our understanding of the excitation-contraction coupling mechanisms in cardiac muscle.